In the field of automation- and process-control-technology, the individual components of a process automation system are generally locationally distributed in the process plant, which makes a communication system necessary for exchange of information between the individual components and/or a control station. Such communication systems for process automation technology must fulfill high technological demands, such as e.g. interface standardizing, Ex-protection, information security, interruption safety, real time applications, simple installation and variable network topology. Considering demands such as these, besides conventional analog field devices with standardized, unidirectional, 4-20 mA electrical current signal outputs, currently also a number of powerful field devices with expanded digital communication capabilities have established themselves. In order to enable a simple integration of these more powerful field devices into existing plants containing 4-20 mA communication, a method has become common, wherein a digitized, high-frequency signal is modulated onto the 4-20 mA electrical current signal for the purpose of carrying additional information. Above all, the HART-standard (HART=Highway Addressable Remote Transducer) has established itself here as a generally valid standard for bidirectional communication between individual field components with the assistance of digital signals. The HART-protocol, which works according to the Bell 202-communications standard using the FSK-method (FSK=Frequency Shift Keying), provides, besides a unidirectional data transmission by 4-20 mA current signal, an additional, bidirectional data transmission using half-duplex communication between, for example, a field device and a control station. Through the supplemental HART-signal, for example, an opportunity has been created for simpler parametering and also for diagnosis of field devices through exchange of status reports.
By adding “intelligence” to the measuring devices in the form of, for example, a microprocessor, multivariable sensors are possible, which, besides ascertaining a primary measured value, ascertain extra measured values, as well as being able to perform diagnostic and analytical functions. Examples of multivariable sensors include transducing flow and viscosity, as measured variables, with a single, Coriolis, flow sensor, and ascertaining pressure difference, absolute pressure and process temperature with a single, pressure sensor. With the help of such multivariable sensor technology, on the one hand, a plurality of different, measured values can be ascertained from the measurement signal, and, on the other hand, additional functions, such as diagnostic functions, maintenance functions, analytical- and evaluating-functions for preprocessing of measured values can be integrated into the multivariable sensor.
A type of embodiment of multilayered communication is that wherein a modulation of a supplemental, digital, HART-signal is effected onto an analog, 4-20 mA, electrical current signal. This has the advantage, that the network topology and older, analog-communicating, field devices can be kept in current communication systems and only small changes must be made in the control system or at the communication partner, since the two different types of communication, i.e. the digital, HART signal and the analog, electrical current signal, do not influence one another. By integrating the ascertaining of different measured variables into a single multivariable sensor, the number of field devices, as well as their process connections, are lessened in a process plant, as a result of which costs for planning, installation, operation and start-up of the multivariable sensors in the process plant turn out to be less. Such multivariable sensors are used in plants running technological methods, such as e.g. occur in the chemical industry, in the pharmaceutical industry, in the foods industry, in the oil and gas industry and/or in the wastewater field.
Also purely digital, fieldbus devices are marketed. An aspect of this is that a large number of different fieldbus systems exist. By way of example, these include PROFIBUS DP, PROFIBUS PA, Foundation fieldbus and CAN. The areas of use of all known bus systems involve special tasks and applications. For this reason, a multitude of field devices exist for various bus systems, and the field devices of one bus system cannot, in general, be compatibly replaced by field devices of another bus system. Fieldbus devices can, it is true, exchange data relatively rapidly via a fieldbus and offer new diagnostic- and monitoring-opportunities; however, their use is limited to, most often, completely newly installed automated plants, in which a new, digital, process control system is provided. This is the reason why the spread of purely digitally communicating fieldbus devices is happening only slowly in process automation technology. A great disadvantage of these purely digitally communicating, fieldbus devices is that they cannot be integrated into existing process plants using analog communication via two-conductor technology.
In order that there be faultless functioning of communication of measured values and diagnostic- and monitoring-values between the intelligent individual components spatially distributed in a process plant, it is necessary that the communication interfaces be matched to one another. For this, such interfaces, as well as further processing mechanisms of the individual field components, are matched by corresponding parametering at start-up.
In order that multivariable sensors can be used in existing process plants having purely analog operating control systems, converters are available, which read-out the digital, HART-signal and convert such into corresponding analog signals in the form of electrical current signals. Such converters include, for example, the TRI-LOOP converter of Rosemount Inc. and the SPA converter of Moore Industries. These converters have the disadvantage that they must first be configured at start-up of the communication system. This has for operators of process plants the disadvantage that additional devices must be configured, and the accompanying inputting of parameters provides an additional source of potential errors.